Measurement of a polarization mode dispersion of optical fibers has been carried out conventionally. For example, Japanese Patent Laid-Open Publication (Kokai) No. H09-264814 describes a polarization mode dispersion measuring device for optical fibers. With reference to FIG. 7, a description will now be given of the device for measuring the polarization mode dispersion of optical fibers according to Japanese Patent Laid-Open Publication (Kokai) No. H09-264814.
First, the polarization mode dispersion τPMD of an optical fiber under test 104 is defined by the following equation (1):τPMD=2√{square root over ({dot over (θ)}2+{dot over (ψ)}12 cos2 θ+{dot over (ψ)}22 sin2θ)}  (1)
where θ is a polarization angle, ψ1 is a phase shift in a certain direction on a plane perpendicular to the propagation direction of light, and ψ2 is a phase shift in a direction orthogonal to ψ1. On this occasion, a transfer function matrix [T] of the optical fiber under test 104 is defined by the following equation (2):
                              [                      T            ⁡                          (              ω              )                                ]                =                  [                                                                                                                                    T                      11                                                                            ⁢                                      ⅇ                                          -                                              jϕ                        11                                                                                                                                                                                                        T                      12                                                                            ⁢                                      ⅇ                                          -                                              jϕ                        12                                                                                                                                                                                                                                T                      21                                                                            ⁢                                      ⅇ                                          -                                              jϕ                        21                                                                                                                                                                                                        T                      22                                                                            ⁢                                      ⅇ                                          -                                              jϕ                        22                                                                                                                          ]                                    (        2        )            
where |Tij| is an amplitude of respective matrix elements, φij is a phase shift of the respective matrix elements, and both of them are functions of an optical angular frequencyω. Then, the parameters –, ψ1, and ψ2 in the equation (1) are respectively obtained by the following equations (3), (4), and (5):θ(ω)=0.5 cos−1(|T11|2−|T21|2)  (3)ψ1(ω)=(φ11−φ22)/2  (4)ψ2(ω)=(φ21−φ12+π)/2  (5)
Consequently, the polarization mode dispersion τPMD of the optical fiber under test 104 is obtained by obtaining the transfer function matrix [T] of the optical fiber under test 104.
A description will now be given of how to obtain the transfer function matrix [T] of the optical fiber under test 104 with reference to FIG. 7. First, a control unit 109 makes output light of a polarization controller 103 as a linearly polarized wave in line with a p direction of a polarization beam splitter 105 incident to the optical fiber under test 104. On this occasion, output light from the optical fiber under test 104 is represented by the following equation (6):
                                          [                                                                                                                                                    T                        11                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          11                                                                                                                                                                                                                              T                        12                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          12                                                                                                                                                                                                                                                          T                        21                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          21                                                                                                                                                                                                                              T                        22                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          22                                                                                                                                          ]                    ⁡                      [                                                            1                                                                              0                                                      ]                          =                  [                                                                                                                                    T                      11                                                                            ⁢                                                            ⅇ                                              -                                                  jϕ                          11                                                                                      ⁡                                          (                                              p                        ⁢                                                                                                  ⁢                        component                                            )                                                                                                                                                                                                          T                      21                                                                            ⁢                                                            ⅇ                                              -                                                  jϕ                          21                                                                                      ⁡                                          (                                              s                        ⁢                                                                                                  ⁢                        component                                            )                                                                                                    ]                                    (        6        )            
The above-described output light is split into an s-polarization component and a p-polarization component by the polarization beam splitter 105, and then, the components are made incident to O/E converters 1061 and 1062 respectively, and the O/E converters 1061 and 1062 respectively measure:|T11|e−jφ11, |T21|e−jφ21
After the above-described measurement, the control unit 109 rotates the output light of the polarization controller 103 by 90°, and makes the rotated light as a linearly polarized wave in line with an s direction in the polarization beam splitter 105 incident to the optical fiber under test 104. On this occasion, output light from the optical fiber under test 104 is represented by the following equation (7):
                                          [                                                                                                                                                    T                        11                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          11                                                                                                                                                                                                                              T                        12                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          12                                                                                                                                                                                                                                                          T                        21                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          21                                                                                                                                                                                                                              T                        22                                                                                    ⁢                                          ⅇ                                              -                                                  jϕ                          22                                                                                                                                          ]                    ⁡                      [                                                            0                                                                              1                                                      ]                          =                  [                                                                                                                                    T                      12                                                                            ⁢                                                            ⅇ                                              -                                                  jϕ                          12                                                                                      ⁡                                          (                                              p                        ⁢                                                                                                  ⁢                        component                                            )                                                                                                                                                                                                          T                      22                                                                            ⁢                                                            ⅇ                                              -                                                  jϕ                          22                                                                                      ⁡                                          (                                              s                        ⁢                                                                                                  ⁢                        component                                            )                                                                                                    ]                                    (        7        )            
The above-described output light is split into an s-polarization component and a p-polarization component by the polarization beam splitter 105, the components are made incident to the O/E converters 1061 and 1062 respectively, and the O/E converters 1061 and 1062 respectively measure:|T12|e−jφ12, |T22|e−jφ22
A network analyzer 107 obtains θ, ψ1, and ψ2 from the respective parameters measured as described above, and the equations (3), (4), and (5). It should be noted that the network analyzer 107 controls an intensity modulation ratio in an optical intensity modulator 102 through an amplifier 108.
Then, the above-described measurement is carried out while the output wavelength of a wavelength-variable light source 101 is being swept, thereby obtaining θ(ω), ψ1 (ω), and ψ2 (ω) from the respective measurement results. Then, the control unit 109 obtains the polarization mode dispersion τPMD from the equation (1).
However, in the method described above, when the output light of the optical fiber under test 104 is separated into the s-polarization component and p-polarization component by the polarization beam splitter 105, the power may be biased. Namely, either the s-polarization component or the p-polarization component output from the polarization beam splitter 105 may be extremely larger than the other component. In even an extreme case, only the p-polarization component is output, and the s-polarization component is not output.
In this case, the S/N ratio of the phase of a component with a smaller power degrades, and the measurement of the phase hence becomes inaccurate. Consequently, a group delay and the polarization mode dispersion of the optical fiber under test 104 contain noise, and the measurement thereof hence becomes inaccurate.
In view of the foregoing, an object of the present invention is to provide a polarization mode dispersion measuring device and the like which prevent the bias in the polarization components.